Electric liquid boiling apparatus having an electronic temperature sensor control

ABSTRACT

An electric liquid boiling apparatus, particularly an electric kettle, having an automatic cut-out operated by an electronic temperature sensor, e.g. a thermistor which is subjected to vapour when the liquid boils.

United States Patent 219/442, 449, 494, 504, 516, 522; 99/280, 281;317/98; 338/22 R, 22 SD Fourny Jan. 8, 1974 1 ELECTRIC LIQUID BOILINGAPPARATUS [56] References Cited HAVING AN ELECTRONIC TEMPERATURE UNITEDSTATES PATENTS SENSOR CONTROL 2,528,191 10/1950 Turner 219/441 [75]Inventor: Denise Gisele Josette Fourny, Monte 3,026,402 3/196 Russell v219/441 Carlo, Monaco 3,539,774 11/1970 Thornton 219 442 2,806,1189/1957 Peterson 219/522 X Assignee= Belling & Limited, Enfield,1,804,130 5/1931 Starr 219/449 Middlesex, England 2,350,941 6/1944Stevenson 219/441 X 3,379,859 4/1968 Marriott 219/522 [22] May 19723,512,909 5/1970 Perkins 1 317/98 x [21] Appl. No.: 250,349 3,577,2095/1971 Perkins 317/98 I Foreign Application Priority Data PrimaryExammerVo1odymyr Y. Mayewsky May 4, 1971 France 7115967 52 us. c1219/441, 99/281, 2191494, [571 ABSTRACT 219/330, 219/5 338/22 R Anelectric liquid boiling apparatus, particularly an [51] Int. Cl F27d11/02 electric kettle, having an automatic cut-out operated 1 1 FieldSearch 441, by an electronic temperature sensor, e.g. a thermistor whichis subjected to vapour when the liquid boils.

8 Claims, 8 Drawing Figures ELECTRIC LIQUID BOILING APPARATUS HAVING ANELECTRONIC TEMPERATURE SENSOR CONTROL The invention relates to electricapparatus for boiling liquids such as, for example, electric kettles,coffee percolators, washing machines, fat friers, etc. The inventionprovides, in such apparatus, electronic means responsive to temperatureand effective to switch off the current supplied to the heating element,when the liquid boils and perhaps also if the heating element overheatsbecause the apparatus is dry.

According to the invention there is provided apparatus for boilingliquids comprising a container for the liquid; an electric heatingelement for heating the liquid in the container; a current-controlledswitch device for breaking the current supply to the heating element; anelectronic temperature sensor situated in a position to be subjected toliquid vapour when liquid in the container boils, and a control circuitconnecting the temperature sensor to operate the switch device to breakthe current supply to the heating element when the temperature sensor isheated sufficiently. As used herein electronic temperature sensor meansa device of which an electrical parameter changes with temperature witha high temperature coefficient. Usually the parameter is resistance andwhat is meant in this context by high temperature coefficient is atemperature coefficient appreciably higher than theresistance/tempcrature coefficient of metals. A thermistor has asuitable temperature coefticient and a device with a temperaturecoefficient half as high as that of a thermistor would be acceptable.Preferably the temperature sensor is a thermistor. The temperaturesensor may have a positive or a negative resistance/temperaturecoefficient.

The temperature sensor may be positioned in the air space above theliquid. However, a feature of the invention consists inpositioning thetemperature sensor in a duct or outside an opening in the containerarranged so that when the liquid boils a vapour stream is ejected fromthe opening or through the duct, the temperature sensor being subjectedto the vapour stream. This arrangement has the advantage that whenboiling ceases, the temperature sensor cools more rapidly than if itwere directly in the air space above the liquid. Thus, the switch devicemay be reset quickly for further operation.

It is often desirable to switch off the electric current to the heatingelement under either of two conditions (a) when the liquid boils and (b)if the heating element should over-heat because the container is dry.The invention may be used to perform either function (a) or bothfunctions. When both functions are to be performed a single temperaturesensor may be used, this being arranged both to be subjected to vapouron boiling so as to be raised in temperature sufficiently by the vapouron boiling and also in thermal contact with the heating element so as tobe raised in temperature sufficiently when the element over-heats. Analternative arrangement when both functions are to be performed is toprovide two temperature sensors electrically connected in tandem so thatheating of either opcrates the switch device, being subjected to vapouron boiling and the other sensor being in thermal contact with theheating element. The second temperature sensor may be another electronicdevice such as a thermistor. Alternatively the second temperature sensormay be another device such as a bimetal strip having contacts connectedto operate the switch device.

In another arrangement in accordance with the invention the boil-drycut-out is a conventional device such as a bimetal strip having its owncontacts which are broken when the container boils dry or peraps beingarranged to trigger a mechanical device which ejects a mains supply plugfrom the container.

The switch device may be a relay and the control circuit may comprise atransistor which controls the energisation of the relay and which inturn is controlled by the temperature sensor or sensors. As analternative the switch device may be a semiconductor device such as athyristor or triac capable of carrying the heating current andcontrolled by the temperature sensor or sensors either directly orthrough the intermediary of a transistor or thyristor.

The control circuit may act simply to cut off th switch device when theliquid has boiled, the circuit or switch device being reset subsequentlyby hand. Alternativ'ely, however, the control circuit may be effectiveto maintain the temperature of the liquid at or slightly below theboiling point by switching off the switch device when the temperaturesensor is-heated and switching the switch device on again when thetemperature sensor cools.

Conveniently, the control circuit derives operating current from themains current supply for the heating element through a transformer orvoltage-dropping impedance.

In a preferred embodiment of the invention the apparatus is an electrickettle for boiling water, the control circuit and switch device beingincludedin the kettle handle. A duct in the handle communicates with theair-space above the water and includes a thermistor which controls amanually resettable relay.

The'invention will further be described with reference to theaccompanying drawings, of which:

FIG.- I is a diagrammatic side elevation of an electric kettle embodyingthe invention;

FIG. 2 is a diagram showing part of the kettle of FIG. 1 with amodification;

FIG. 3 is a diagram showing part of the kettle of FIG. 1 with anothermodification;

FIG. 4 is a circuit diagram of the kettle of FIGS. 1-3;

FIG. 5 is a circuit diagram of an alternative circuit for use inaccordance with the invention; and

FIGS. 6, 7 and 8 are diagrams illustrating possible modifications to thecontrol circuit.

Referring to FIG. 1 there is shown an electric kettle comprising a body1 at the bottom of which is an electric heating element 2. Electriccurrent for the heating element is derived from a socket 3 throughcontacts 4 of a relay 5. The handle 6 of the kettle houses the relay 5and also a control circuit 7 therefor. The control circuit is controlledby a thermistor 8 which is situated in a duct 9 in the handle. The duct9 communicates at 10 with the air-space above the water and at 11 withatmosphere.

In use, the kettle is filled with water and connected by a plug and lead(not shown) to the mains supply. A push-button 12 is depressed and thisvcloses the contacts 4 of relay 5 directly. The relay is energised andholds contacts 4 closed. When the water boils steam passes through duct9 and heats thermistor 8 to a temperature of about C. This is sufficientto'cause the control circuit 7 to de-energise relay 5, thus causing thecontacts 4 to open and cut off the current supply to the heatingelement.

FIG. 2 shows a modification of the kettle of FIG. l. The kettle of FIG.2 is the same as that of FIG. 1 except that another thermistor 80 hasbeen added. Therrnistor 8 is not shown in FIG. 2 although it is present.Thermistor 8a is effective instead to cut off the current supply if theheating element over-heats and is mounted on a beryllium-copper strip 13which is soldered to the heating element 2. The strip 13 conducts heatfrom the heating element to the thermistor and maintains the thermistorat substantially the temperature of the heat ing element. The circuit issuch that if the temperature of the heating element rises significantlyabove the boiling point of water to 125C, say, then relay 5 isdeenergised to open contacts 4. It will be seen that thermistors 8 and8a are effectively connected in parallel to de-energise the relay ifeither is heated sufficiently. Thus, this embodiment provides cut-off ofthe heating element current if the water boils or if the heating elementover-heats.

Another way of performing both functions is illustrated in FIG. 3. Inthis arrangement a single thermistor 8 is mounted on a beryllium-copperstrip 13 as in FIG. 2 and is also situated in a duct 9 communicatingwith the air-space above the water. Thus the thermistor will be heatedsufficiently to de-energise the relay if the water boils or if theheating element over-heats.

FIG. 4 is the circuit diagram for any of the kettles of FIGS. 1 to 3.The alternating current supply is shown at 14. Heating element 2 isconnected across the supply when relay contacts 4 are closed, as is aseries combination of a voltage-dropping impedance is and a fullwavebridge rectifier 16. The output from rectifier I6 is smoothed by acapacitor 17 and applied to the relay coil 5. It will be seen that whencontacts 4 are closed by depression of push-button 12 (FIG. I) the coil5 is energised and holds contacts 4 closed.

A transistor 18 is connected across coil 5 and is normallynon-conducting. However, thermistor 8, which in this embodiment has anegative resistance/temperature coefficient, is connected in thebase-emitter path of the transistor. When the temperature of thermistor8 rises sufficiently its resistance drops to a level which allowstransistor 18 to conduct and thereby short-circuit relay 5. The relay isthus de-energised and the contacts 4 open to cut off the current to theheating element.

In thearrangement described above in which two thermistors are provided,a suitable method of connecting'the second thermistor is in parallelwith the first, as indicated at 8a in FIG. 4.

FIG. 5 shows an alternative circuit diagram for kettles such as those ofFIGS. 1 to 3. Instead of a transistor in parallel with coil 5 there isprovided a transistor 19 in series with it. The transistor is normallyconducting and is cut off by sufficient increase in the resistance ofthermistor 8 which has a positive resistance/temperature characteristicand is connected to the base of the transistor. A resistor 20 isprovided to act as a load for the rectifier 16 when the transistor iscut off. For the sake of illustration FIG. 5 shows an arrangement inwhich energisation of the control circuit does not depend upon closureof contacts 4.

FIG. 6 shows an alternative disposition for the thermistor 8 in relationto the transistor 18 of FIG. 4.

FIG. 7 shows that the transistor 18 can be p.n.p. instead of n.p.n. andshows a suitable arrangement for a thermistor 21 with a positiveresistance/temperature characteristic.

FIG. g shows a further modification where, instead of a transistor 1%across coil 5 there is provided a thyristor 18a, the trigger electrodeof which is connected to the thermistor 8 so that the thyristor istriggered to conduct when the temperature of the thermistor reaches apredetermined level.

The invention is not restricted to the details of the embodimentsdescribed above with reference to the drawings. For example, instead ofrelay Sand contact 4 there may be provided a semiconductor triggerarrangement such as a thyristor; a thyristor pair or a, triac capable oftaking the current for the heating element. This device would berendered normally conductive by operation of a manual push button andwould be cut off by sufficient heating of thermistor 8 connecteddirectly or by way of a transistor to the trigger electrode of thesemi-conductor trigger arrangement.

The rectifier 16 may be a half-wave rectifier and capacitor I7 isoptional, although it is preferably provided if the rectifier is a half.wave rectifier.

The temperature at whichcut-off takes place is ad,- justable byvariation of the circuit parameters. A vari able resistor may beincluded in the circuit to allow the temperature to be set at will.

The temperature sensor need not bea thermistor but may be, for example,a semiconductor diode or perhaps a transistor.

In the above described embodiment in which there are both thermistors 8and 8a provided, these thermistors may be electrically identical. Thisis possible in spite of the fact that they should operate underdifferent circumstances because of the difference in heat flow.Thermistor 3 which is subjected to the vapour is heated and cooled muchmore quickly than thermistor 8a. However, if necessary the thermistorsmay be different electrically.

The claims defining the invention are as follows:

I. In an apparatus for boiling liquid comprising a container for theliquid, said container having an opening therein through which hotliquid vapor is ejected when the liquid boils, an electric heatingelement in the container for heating the liquid therein and means forcarrying electric current to said heating element, the improvementcomprising a current-controlled switch device in series with saidcurrent carrying means; a control circuit having an input and an output,said output being connected to'the current controlled switch device; andan electronic temperature sensor means having a high temperaturecoefficient positioned so as to be subjected to said ejected hot liquidvapor when the liquid in the container boils and being electricallyconnected to said input of said control circuit, whereby when the liquidboils, the electronic temperature sensor is heated and the controlcircuit operates the current-controlled switch device to cut off theheating current for the heating element.

2. Apparatus as claimed in claim 1 wherein the temperature sensor meansis a thermistor.

3. Apparatus as claimed in claim 1 wherein the temperature sensor meansis situated outside said opening adjacent thereto.

4. Apparatus as claimed in claim I including a thermally conductivemetal strip in thermal contact with said heating element and saidelectronic temperature sensor means, whereby said switch device isoperative to break the current supply to the heating element both whenthe liquid boils and when the heating element overheats.

5. Apparatus as claimed in claim 1 including a second electronictemperature sensor means, said second sensor means being in thermalcontact with said heating element, the two temperature sensor meansbeing electrically connected in parallel so that heating of eitheroperates the switch device, whereby said switch device is operative tobreak the current supply to the heating element both when the liquidboils and when the heating element overheats.

6. Apparatus as claimed in claim 1 wherein the switch device is anelectromechanical relay which is manually resettable.

7. Apparatus as claimed in claim 6 wherein the control circuit comprisesa transistor having an emittercollector path connected to control theenergization of the relay and a base emitter path connected to the saidinput to be controlled by said temperature sensor means.

8. Apparatus as claimed in claim 1 wherein said container is a kettlefor boiling water, said kettle including a handle, the control circuitand switch device being included in said handle, a duct in the handlecommunicating with said opening and with the air-space above the water,said duct having the temperature sensor means positioned therein.

1. In an apparatus for boiling liquid comprising a container for theliquid, said container having an opening therein through which hotliquid vapor is ejected when the liquid boils, an electric heatingelement in the container for heating the liquid therein and means forcarrying electric current to said heating element, the improvementcomprising a current-controlled switch device in series with saidcurrent carrying means; a control circuit having an input and an output,said output being connected to the current controlled switch device; andan electronic temperature sensor means having a high temperaturecoefficient positioned so as to be subjected to said ejected hot liquidvapor when the liquid in the container boils and being electricallyconnected to said input of said control circuit, whereby when the liquidboils, the electronic temperature sensor is heated and the controlcircuit operates the current-controlled switch device to cut off theheating current for the heating element.
 2. Apparatus as claimed inclaim 1 wherein the temperature sensor means is a thermistor. 3.Apparatus as claimed in claim 1 wherein the temperature sensor means issituated outside said opening adjacent thereto.
 4. Apparatus as claimedin claim 1 including a thermally conductive metal strip in thermalcontact with said heating element and said electronic temperature sensormeans, whereby said switch device is operative to break the currentsupply to the heating element both when the liquid boils and when theheating element overheats.
 5. Apparatus as claimed in claim 1 includinga second electronic temperature sensor means, said second sensor meansbeing in thermal contact with said heating element, the two temperaturesensor means being electrically connected in parallel so that heating ofeither operates the switch device, whereby said switch device isoperative to break the current supply to the heating element both whenthe liquid boils and when the heating element overheats.
 6. Apparatus asclaimed in claim 1 wherein the switch device is an electromechanicalrelay which is manually resettable.
 7. Apparatus as claimed in claim 6wherein the control circuit comprises a transistor having anemitter-collector path connected to control the energization of therelay and a base emitter path connected to the said input to becontrolled by said temperature sensor means.
 8. Apparatus as claimed inclaim 1 wherein said container is a kettle for boiling water, saidkettle including a handle, the control circuit and switch deviCe beingincluded in said handle, a duct in the handle communicating with saidopening and with the air-space above the water, said duct having thetemperature sensor means positioned therein.